Prior art fluoroscopic imaging systems use x-ray tubes that are intermittently powered by application of pulses thereto, which tubes generate the x-ray beams directed through the inwards of a living creature being observed to a screen of phosphorescent material or to any array of scintillators. The pattern of x-ray penetration through the creature being observed is thereby converted to a light pattern. Optics focus the light pattern to generate an image within a video camera. Each pulse of x-ray beams generates a respective image for the video cram, which image is scanned by the electronic camera device to generate a respective frame of video input signal for a video monitor. A video monitor is like a television receiver without the front end portions used to detect video signal from radio waves. The video monitor requires a prescribed regular frame rate for the video signals it receives if, as is the usual practice, the video monitor uses a kinescope with electromagnetic deflection and the electromagnetic deflection uses resonant energy recovery methods. The pulse rate for gating the x-ray tube into operation has been chosen high enough that the regular rate of frames generated by the electronic camera device is sufficiently high to avoid excessive flicker in the average brightness level of the visual display generated by the video monitor. The flicker in the average brightness level can be reduced by using longer-persistence phosphors in the viewing screen of the video monitor, but smear of moving objects becomes noticeable with long-persistence phosphors. The human observer is more sensitive to flicker at higher light levels, but when using normal-persistence kinescope phosphors a 60 Hz flicker rate is deemed to be unobjectionable at normal room light levels when viewing broadcast television (which is raster scanned interlacing the scan lines of alternate fields). Somewhat more display flicker is tolerated in fluoroscopy than in broadcast television, progressive scanning at 30 Hz frame rate being a customary practice in fluoroscopy employing a video camera. The display frame rate is a fixed rate to accommodate resonant energy recovery in the electromagnetic deflection circuitry for the kinescope in the video monitor.
It is known in the prior art that the introduction of a frame storage device between the electronic camera device and the video monitor can avoid x-ray exposures having to be made at the display frame rate in order that the electronic camera device can generate consecutive frames of image samples for the video monitor at a display frame rate high enough to avoid flicker in the average brightness level of the video monitor. Each frame of video camera samples generated during a respective one of less frequent x-ray exposures is written to the frame store in a procedure called "frame grabbing". The writing of the frame store is performed in synchronism with the raster scanning of the video camera, during which frame the image samples are supplied directly to the video monitor. The frame of image samples stored in the frame store can then be repeatedly raster scanned to supply repeated frames of image samples until an updating frame of image samples is available from the video camera responsive to the next x-ray exposure.
Accordingly, the use of the frame store facilitates lowering on average the pulse rate of gating the x-ray tube into operation, which reduces the dosage of x-rays to which the living creature being observed is exposed over a given period of time. With the use of the frame store, the reduction of this pulse rate is limited by the tolerance of a human observer to motion flicker, or the stroboscopic effect of reduced field rate, in the video display on the video monitor screen. Motion flicker becomes more noticeable as motion becomes faster, so the degree of reduction of the pulse rate for gating the x-ray tube operation is limited as a function of the rate of motion in the portion of the inwards of the living creature being fluoroscoped. Accordingly, in practicing their method of x-ray dose reduction, the inventors make available a range of pulse rates that can be keyed to the nature of the organ(s) being observed by their fluoroscopic system.
The inventors rely on frame filling apparatus to reduce the motion flicker that occurs when the pulse rates for gating the x-ray tube are lowered, so that a greater reduction of the pulse rate for gating the x-ray tube can be tolerated and the average dose of radiation over a given time can be reduced still more. A. Abdel-Malek, O. Hasekioglu and John Bloomer describe a feasibility study they made with regard to reducing pulse rates for gating the x-ray tube in fluoroscopic apparatus in their paper "Image segmentation via motion vector estimates", SPIE Vol. 1233 Medical Imaging IV: Image Processing (1990), pp. 366-371, published 6 Feb. 1990. This feasibility study (incorporated herein by reference) found that acceleration-compensated interpolation techniques provided superior estimates of fill frames. The feasibility study was made processing sequences of recorded images in extended time. Arranging for frame-filling techniques to reduce the motion flicker that occurs when the pulse rates for gating the x-ray tube are lowered, which techniques meet the requirements that they be executed in real time, presented a substantial technical challenge.